Automotive vehicles have realized the efficient run by an onboard automatic transmission which delivers a shaft torque of an internal combustion engine (or called “engine”) to its output side propelling traction wheels while adjusting the number of rotations to the suitable one. This automatic transmission employs the detected temperature from an outside air temperature sensor to provide efficient transmission of power because it is necessary to adjust control conditions in response to outside air temperature and so forth.
It is the common practice to place such outside air temperature sensor in an engine compartment. Thus, the outside air temperature sensor is under the influence of radiant heat from the internal combustion engine, so that it cannot be expected that the outside air temperature sensor always detects outside air temperature appropriately. As one approach to remedy the influence of radiant heat, it is proposed by JP-A 2009-228773 to estimate an end-point temperature (or outside air temperature) of automatic transmission fluid.
In a ratio shift control system for an automatic transmission described in the above-mentioned JP-A 2009-228773, the measurement values of the engine coolant temperature at a current engine startup event and the previous event and the time elapsed from the previous event to the current event are employed to calculate a final end-point temperature (outside air temperature) as an estimate for the outside air temperature.
However, the engine coolant temperature, which varies depending on the engine running, does not completely match the fluctuation in outside air temperature. Thus, adjusting an operating condition of an automatic transmission according to the estimate (for outside air temperature) makes it difficult to provide an efficient and high quality drive control.
Such estimation may cause an inaccurate adjustment of the operating condition when an operating environment differs greatly, for example, from an intense cold region to an intense heat region between the previous event and the current event.
Incidentally, it might be possible to employ the detected temperature of an outside air temperature sensor as it is, provided that the engine coolant temperature has dropped to or below a cold engine evaluation threshold, because the outside air temperature sensor is placed in a so-called engine compartment together with an internal combustion engine and it is under the influence of radiant heat that is emitted according to an amount of time during which the internal combustion engine is in motion. Moreover, it might also be possible to employ a lower one of a current value detected at a current time and the previous value detected at the previous time when the engine coolant temperature exceeds the cold engine evaluation threshold.
However, when the operating environment differs greatly, for example, from the intense cold region to the intense heat region between the previous event and the current event, this control strategy would result in utilizing the temperature previously detected by the outside air temperature sensor in the intense cold region though it should have utilized the temperature detected by the outside air temperature sensor at the current event in the intense heat region. This makes it difficult to provide an efficient and high quality drive control.
If, taking such intense heat into consideration, the cold engine evaluation threshold were set high beforehand in order to solve this problem, the present temperature detected by the outside air temperature sensor under heavy influence of the radiant heat would be employed rather than the previously detected temperature closer to the actual outside air temperature. This makes it difficult to expect realization of an efficient and high quality drive control.